Compressor construction



Patented Mar. 21, 1950 COMPRESSOR CONSTRUCTION Nathan C. Price, St.Helena, Calif., assigner to Lockheed Aircraft Corporation, Burbank,Calif.

Application November 28, 1947, SerialNo. 788,350

This invention relates to compressors or blowers and has more particularreference to axial flow type compressors. It-is a. general object of theinvention to provide an axial ow compressor characterized by a simple,inexpensive and extremely light weight rotor construction.

'I'his application is a continuation-in-part of my co-pendngapplication, Serial No. 583,152, led March 16, 1945, which, in turn, isa division of my co-pending application, Serial No. 488,029, filed May22, 1943, now Patent No. 2,468,461, issued April 26, 1949.

23 Claims. (Cl 230-122) It is another object of the invention to providean axial blower or compressor of the axial flow multiple blading rowtype which has an extremely light weight rotor that preserves itscircularity and maintains adequate blade tip clearance even during highspeed operation. The light weight rotor particularly adapts thecompressor for use in aircraft power plants, and the like, where weightis an important consideration.

Another object of the invention is to provide a compressor of the classreferred to characterized by a light weight rotor shell of thin metaleffectively reinforced by internal hoops which.

carry the axially spaced rows of radially projecting blades to relievethe shell of the major loading. This form of rotor construction is wellsuited for mass production methods, is inexpensive, and results in astrong, dependable, light weight wheel or rotor.

Another object of the invention is to provide an axial flow compressorin which the rows of impeller blades in upstream and downstream relationto one another are slotted and have their interiors in communicationwith one another so that the pressure differential in the main passageoi' the blower causes air flow into the slots of the blades in one rowand the discharge of air from the slots of the other row to increase theboundary layer flow over the surfaces of the blades thereby assuringeilicient blade operation throughout the wide range of coefllcients oflift that may be encountered as a result of variation in ambient airdensity and pressures at the rst rows of diffuser vanes. vThe augmentedboundary layer flow reduces the tendency of the boundary layer toseparate from the blade surfaces and lessens the drag so that the mainair flow follows the blade surfaces to greater angles of attack.

When the compressor is embodied in a power plant for propelling highaltitude aircraft, it must, of necessity, operate at varying pressurealtitudes and function at variable compression ratlos. stant operativepressures within the combustion zone and gas turbine of the power plant,the first stage compressor is regulated to operate at lower speeds inthe denser ambient air of low altitudes and to function at higher speedsin the low density ambient air of the higher altitudes. This variationof operating conditions requires that the impeller blades of the rststage compressor and, particularly,- the rst rows of such blades operatethrough a wide range of angles of attack relative to the entering airand with respect to the partially compressed air leaving the' first rowsof intermediate diffuser vanes, that is the initial rows of impellersmust operate over a wide range of coeflicients of lift. This range ofcoeflicients of lift is greatly in excess of the range for emcientimpeller operation and under certain conditions at low altitude the rstrows of impeller vanes may operate under practically stalled conditions.The increased boundary layer ow obtained by the present invention bringsabout eiiicient vane or blade operation through the wide range ofcoeilicients of lift.

It is a further object of the invention to provide an axial compressorof the class referred to in which the hollow rotor shell constitutes acommon means of communication between the several hollow slotted bladeswith appropriate metering means being provided for the blades to assurethe required or proper relationship between the Volume of air flowthrough the slots of the spaced blading rows.

Other objectives and advantages of the invention will become apparentfrom the following detailed description of typical preferredembodiments, throughout which description reference is made to theaccompanying drawings wherein:

Figure l is a fragmentary side elevation of a compressor `of theinvention with a portion broken away to illustrate the rotor andadjacent parts in cross section;

Figure 2 is an enlargedfragmentary sectional view of the compressorshell showing the blades in side elevation;

Figure 3 is a fragmentary sectional View taken as indicated by lines 3 3of Figure 2;

Figures 4 and 5 are enlarged transverse sectional views of the blades ofdifferent rows;

Figure 6 is a fragmentary sectional view of another form of bladeattachment; and

Figure 7 is a fragmentary perspective view oi a blade and blade shankemployed in the structure of Figure 6.

In the drawings the invention is illustrated In order to maintainsubstantially conthe impeller varies.

embodied in a blower or compressor for an internal combustion turbopower plant of the type disclosed in my co-pending application SerialNo. 488,029. The compressor may be considered the first stage compressorarranged at the forward end of the power plant to receive the ram airand may be considered as driven by the turbine of the power plantthrough the medium of a suitable drive. However, it will be apparentthat the features of the invention may well be embodied in compressorsdesigned for other applications and installations.

The illustrated structure includes an elongate cylindrical housing ||lwhich may constitute the forward portion of the power plant casing andthe case or shell of the compressor. The forward end of the housing l isopen and provided with a grooved spigot |2 which constitutes theinlet'of the compressor. v

The blower or compressor includes a rotor arranged longitudinally andcoaxially within the housing III. The construction of the wheel or rotoris a feature ofthe invention. The improved rotor comprises a shell 20preferably constructed of relatively thin sheet metal or the like. Whilethe thickness of the sheet metal of the rotor shell will depend upon theoverall size of the compressor, and upon other considerations, thethickness of the shell in most instances is between thirty-five andninety onethousandths of an inch. The rotor shell 20 is in the form of atruncated cone of rearwardly increasing diameter and may be fabricatedfrom a thin metal tube spun or otherwise-'worked to the required shape.The compressor rotor further includes internal rings or hoop-likereinforcing and blade carrying elements. In the construction illustratedthere is a plurality of axially spaced pairs of reinforcing rings 2| and22 of angular cross section. The pairs of rings 2| and 22 are graduatedor varied in diameter to conform to the tapering shell 2l and theiraxially extending flange parts are fixed to the internal surface of theshell by riveting, welding, braining. or the like. As best illustratedin Figure 2 the hoops or rings of each pair are spacedl apart axially ofthe rotor and are related to have their inwardly projectingcircumferential flanges i3 in spaced generally parallel relation. Thepairs of rings 2| and 22 are spaced longitudinally throughout the lengthof .the shell 20 or at least throughout that portion of the shell whichcarries The rings 2| and 22 mount the impeller vanes as will be laterdescribed and their spacing is determined by the required spacing of theblading rows.

The rotor shell 20 constructed of the `thin sheet metal and reinforcedby the spaced internal hoops or rings 2| and 22,-.as just described, islight in weight and inexpensive to manufacture. and yet possesses amplestrength and rigidity to maintain its circularity and the blade tipclearance during high speed operation.

I have shown the forward end of the rotor shell 20 provided with aforwardly projecting h ollow spindle 48 rotatably supported in suitablebearings 49. The bearings 49 are mounted within a streamlined housing 50centrally suplported in the compressor inlet by spaced radial vanes 5|of the spigot I2. The vanes 5| may be set at an angle of incidence withrespect to the axis of the compressor to impart an initial swirl to theair entering the compressor. The

direction of this swirl is counter to the direc-y tion of rotation ofthe rotor 20 to increase the A efficiency of the initial compressionstage. The

rear portion of the rotor 20 is suitably supported in the housing lil orthe power plant case by supporting and driving means not shown.

The rotor 20 of the blower carries a plurality of axially spaced rows ofimpeller blades 25. In accordance with this invention, the blades 25 arehollow and are preferably constructed from relatively thin sheet metal.The blades 25 are closed at their tips except for small vent holes 85,and are provided at their root ends with metal Shanks or fittings 24.The vent holes 85 are for the purpose of preventing accumulation of oilwithin the blades, whichmight occur over a period of time d-ue toseepage past bearing seals. The fittings 24 are separately formedelements engaged within the root portions of the hollow blades at 21 andsecured therein by welding or furnace brazing. The portions of thefittings 24 which project beyond the ends of the blades are reduced incross section leaving shoulders 23 flush with the root ends of theblades. The reduced tongues of the fittings 24 are passed throughcorrespondingly shaped slots or openings 9 in the wall of the rotorshell. The openings 9 are provided in spaced circumferential rows andare located so that the inwardly projecting fittings are receivedbetween the inwardlyl extending flanges of the adjacent rings 2| and 22.These projecting parts of the fittings 24 are employed as anchor lugsfor the blades. Each fitting 24 has a transverse opening 3 receiving abolt or pin 26. The pins 26 serve to secure the fittings to the adjacentrings 2| and 22. The parts are related so that the shoulders 23 at theroot ends of the blades 25 engage and conform to the external surface ofthe rotor 20 when the pins 26 are engaged in their openings 3 to ilx theblades to the mounting rings. With this mode of attaching the blades 25the pairs of hoops or rings 2| and 22 carry the concentrated centrifugalforces of the blades, relieving the shell of the major loads so that theshell may be fabricated from relatively-thin light sheet metal forminimum weight. Furthermore, ther blade attaching means assures uniformspacing of the blades and facilitates fabrication by mass productionmethods.

In the form of construction illustrated in Figures 1, 2 and 3, the rootfittings 24 are ported or provided-with passages to maintain theinteriors of their respective blades 25 in communication with theinternal cavity of the hollow rotor 20. Thus, as shown in Figure 3, theshank fitting 24 has a pair of ducts 29 and 30 leading from the interiorof the respective blade 25 to the interior of the hollow rotor. Theblades 25 of the several forward rows of blading are designed to providefor the circulation air therethrough, thereby effecting boundary layercontrol action for the blades. It is contemplated that boundary layercontrol apertures or slots of various configurations and in diierentlocations may be provided in the blades. However, the invention providesan improved arrangement of slots for effecting a most efilcient boundarylayer control action throughout substantially the entire lengths of theblades.

The bodies of the blades 25 in the several forward rows are eachprovided with a plurality of relatively narrow longitudinally extending-slots 3| to 34. The slots 3| to 34 are provided in the convex oranti-lift walls of the blades ln-or ad\ jacent the chordwise portion ofmaximum thickness. As shown in Figures 4 and 5, the substantiallyairfoil-shaped blades 25 are designed so that their portions of maximumthickness are forward of the medial axial planes, and the slots ting ofthe blade walls throughout practically the entire lengths of the bladeswithout materially weakening the blades or lessening their rigidity.

The number of forward rows of impeller blades 25 provided with slots 3|to 34 as just described, will vary in blowers intended to function undererent operating conditions. In most instances it will be foundsufficient to provide the slots in the three or four initial rows ofblading. Figure l shows the three forward rows of blades 25 providedwith the slots 3| to 34, it being understood that this is merelytypical. It is preferred that the slots in certain rows be forwardlydirected for the inspiration-or the reception of air, while th'e slotsof other rows be rearwardly directed for the discharge or expiration ofair. Where there are three rows of slotted blades, as illustrated, it ispreferred to make the slots in the two rearward rows forwardly directedfor the reception or inux of air and to make the slots in the forwardrow, rearwardly directed for the discharge or efflux of the air. Theblade walls at the forwardly directed slots 3| to 34 may have internallips 1 while the blade walls at the rearwardly directed slots 3| to 34'may have internal v lips 6, the lips 6 and 1 serving to assist indirecting the air and to give the slots a-substantially nozzle shape. Asabove described, the ducts 29 and 30 in the blade root fittings 24,maintain the interiors of the blades 25 in communication with theinternal cavity of the hollow rotor so that the air received by theslots of the said two rearward rowsof bladesenters the interior of therotor and passes forwardly therein to the forward row of blades fordischarge from the slots thereof. Thus the cavity of the hollow rotor 20forms a common means of communication between the interiors of theseveral rows of hollow blades. With this simple construction, theslotted blades are maintained in communication with one another withoutthe necessity of providing special ducts or passages.

The invention provides means for distributing air flow through theslotted blades to obtain a balanced or desired boundary layer controleffect at the blades of the several rows. It is contemplated that thewidth or capacity of the slots 3| to 34 of slotted blading rows berelated to obtain the desired proportionate or balanced boundary layercontrol action, or the ducts 29 and 30 of the several blading rows maybe proportioned in so far as capacity is concerned to balance the airflowv through the several rows. Furthermore, the capacity of both theslots and the ducts may be controlled or related to bring about apreferred relationship of air flow. Unless such provision is made for adistributed ilo-w of boundary layer control air there may be asubstantially neutral condition in the intermediate row or rows ofslotted blades, with little or no air flow therethrough.

rows. Thus, where there are three rows of slotted blades as illustrated,the root ducts 29 and 3l of the blades in the third or rearward row areconstricted to bring about a substantially balanced or equalized airflow through'the second and third rows of blades Air will ow into thesecond and third rows of blades and pass out through the initial row ofblades.

A plurality of rows of radial inwardly extending diffuser orcountervanes 58 is provided on the inside of the housing I0. Thestationary vanes 58 are arranged to stand with small clearance betweenthe impeller fblades 25 of the rotor 20.

Figures 6 and 7 illustrate another form of rotor construction and bladeattachment of the invention. In this case the rotor shell 20a may be asimple thin walled sheet metal member of the desired configuration as inthe previously described embodiment. Axially spaced rings 19 areprovided on the internal surface of the shell 2|! to reinforce the sameIand to carry the rows of blades 12. y

The rings 10 correspond in function to the above described rings 2| and22.and are suitably fixed to the rotor shell 20a by riveting, spotwelding, brazing or the like. However, in this case there is a singlering 10 at each row of impeller blades 12 and each ring 10 is somewhatchannelshaped in transverse cross section having two spaced flanges 1|which project inwardly toward the axis of rotation of the rotor. Thehoops or rings 10 have a wall thickness at the wall of the shell 20aapproximately the same as the thickness of the shell wall to facilitatethe welding or securing of the rings to the shell. The flanges 1| areprovided at their inner or opposing sides with shoulders 14. Theshoulders 14 of the sets of flanges 1| are directly opposite or incommon plane and face inwardly toward the axis of rotation of the rotor.The rings 10 are further provided with circumferentially spaced radialopenings 13 for receiving the shanks of the blades 12. The openings 13are centrally spaced struction as the blades 25 described above, beingsI have found it desirable to employ the root ducts l 29 and 30 asmetering passages. For example, the ducts 29 and 39 of certain bladingrows are constricted with relation to the ducts of the other between thesidesof the rings 19 to -join their channels and have radially outwardfacing seats 15. The seats 15 are flat and perpendicular to the axes ofthe openings 13 but have llets or radii where they join the openings 13.Openings 16 are formed in the wall of the rotor shell 29a `to registerwith the seats 15 and to be coaxial with the openings 13. The openings13 and 16 and the seats 15 are round or cylindrical and are equallyspaced about the circumference of the rotor 29a in accordance 'with theprecalculated location of the blades 12.

The blades 12 may be of the same general conhollow formed sheet metalelements of the required configuration. Each blade 12 is provided with aseparately constructed shank tting having a shank 11 for engaging in anopening 13, and an intermediate circular or .cylindrical head 'I8 forreception in an opening 16 and for engaging a seat 15 and a transversetongue 19 to' be secured ina blade. The Shanks 11 are cy-lindrical toconform with the openings 13 of the rings 10 and their inner ends areadjacent the shoulders 14 of -the'rings. The heads 18 are fiange-likeparts presenting flat inner surfaces for cooperating with the seats 15to establish the perpendicularity of the blades with respect to therotor. The tongues 19 of the shank fittings are shaped to conform to theinternal configuration of the blades 12 and in the constructionillustrated are substantially airfoil shaped to-correspond with the-blade shape. The shank fittings are preferably precision castings inwhich case the tongues 19 require no -machining and the shanks 11 andheads 18 need only slight machining. The hollow blades 12 are securelyfixed or bonded to thetongues 19 of their shank llttings, for examplethe blades and tongues may be furnace bonded. Thus the shear areas ofthe blade assemblies are securely joined to dependably withstand theoperational loading. In practice it may be found advantageous toconstruct the blades 12 and the shank fittings, or at least the tongues19 of `the ttings, of the same material to assure a good bonding ofblades and tongues. The inner end or base surfaces of the blades 12 andtheir tongues 19 are ground or otherwise machined to substantially fitthe external surface of the rotor shell 20a.

The blades 12 are secured to the rotor assembly by cap screws 88 screwedinto bores in the inner ends of the Shanks '.11 so that their headscooperate with the above described shoulders 14. It is preferred toarrange washers 8l between the shoulders 14 and the heads of the capscrews 80. It will be seen that Awhen the blades 12 have been set at theproper angle, the screws 80 maybe tightened down to secure the blades inposition. The cap screws 80 may be locked in place by safety wires 82.Pins 88 may engage in openings in the rings and shank fitting heads 18to preserve the angular setting of the blades 12. The cap screws 80 arepreferably tubular and where desired or necessary, ports or air passages83 extend through the tongues 18 and heads 18 to communicate with thetubular screws so that the interiors of the blades 12 and rotor shell20a are connected for the circulation of boundary layer air as in thepreviously described form of the invention.

From the foregoing it will be seen that I have provided a blower orcompressor rotor construction that is extremely simple and inexpensiveto manufacture and that islight in weight. The

exceptionally small weight of therotor well suits it for embodiment inaircraft power plants and. of course, minimizes centrifugal forces andinertia effects. While the construction is light in weight, thereinforcing hoopsor rings 2l and 22 within the housing, a plurality ofrows of hollowy blades projecting from the rotor and having fluidapertures in their walls, the interiors of the blades being incommunication with the internal cavity of the hollow rotor so that fluidmay flow -from one blade to another through said cavity,

and means for restricting the flow through the blades of certain rows tosubstantially equalize the flow through the blades of the several rows.

2. An axial blower comprising a housing, a ro- -tor in the housingincluding a hollow thin walled shell, and internal hoops reinforcing theshell, hollow impeller blades projecting from the shell and anchored tosaid hoops, the blades having communication with the interior of theshell, and

means for producing air flow through the shell hollow rotor shell,internal reinforcing hoops for the shell, hollow blades for vprojectingfrom the rotor, and fittings secured to the roots oi' the blades andattached to said hoops to mount the blades on the rotor.

5. A rotor for an axial blower comprising a thin walled hollow rotorshell, internal reinforcing hoops on the wall of the shell, hollowblades for projecting from the rotor, fittings fixed in the rootportions of the hollow blades, and means for securing the fittings tosaid hoops to attach the blades to the rotor.

6. A rotor for an axial blower comprising a hollow rotor shell, internalreinforcing hoops for the shell, the hoops having inwardly projectingcircumferential flanges, blades for projecting from the rotor, rootparts on the blades received between said ilanges, and means forattaching said paots to the flanges to secure the blades to the ro r. l

'7. A rotor for an axial blower comprising a hollow rotor shell,internal reinforcing hoops for the shell, the hoops having inwardlyprojecting circumferential flanges, blades for projecting from therotor, root parts on the blades received between said flanges, and meansfor attaching said parts to the flanges to secure the blades to therotor, including pins received in axial openings in the flanges andparts.

8. A rotor for an axial blower comprising a hollow rotor shell, internalreinforcing hoops for the shell, hollow blades for projecting from therotor, root fittings brazed inthe hollowv blades, and means for securingthe fittings to said hoops.

9. A rotor for an axial blower comprising a hollow rotor shell, internalreinforcing hoops for the shell, hollow blades for projecting from therotor, ttings projecting from the root ends of the blades and secured tosaid hoops, there being ducts in the fittings placing the interiors ofthe blades in communication with the interior of the shell and boundarylayer slots in the blades arranged to cause the flow of air throimh theblades and the interiorof the shell.

10. A rotor for an axial blower comprising a hollow rotor shell,internal reinforcing hoops for the shell, hollow blades for projectingfrom the rotor, fittings projecting from the root ends of the blades andsecured to said hoops, there being ducts in the fittings placing theinteriors' of the blades in communication with the interior of the shelland boundary layer slots. in the blades arranged to cause the flow ofair through the blades and the interior of the shell, said ducts ofcertain blade fittings being restricted to meter the flow of air throughsuch blades.

11. A rotor for an axial blower comprising a hollow rotor shell,internal hoops on the wall of the shell, hollow blades for projectingfrom the rotor, fittings projecting from the root ends oi' the bladesand secured to said hoops, therebeing ducts in the fittings placing theinteriors of the blades in communication with the interior of the shelland boundary layer slots in the blades, said slots of downstream bladesbeing forwardly directed for the influx of air and the slots ofupthin-walled light weight hollow shell having rows of circumferentiallyspaced openings, circumferential internal stiffening rings on the shell,and hollow blades secured to the rings and extending through saidopenings to projecti'from the shell.

.the assembly of the shell, rings, and blades constituting a structurehaving great liexural rigidity and minimum inertia.

13. In an axial compressor, a hollow sheet metal rotor shell having arow of circumferentially,

spaced openings, a ring secured to the interior of the shell at said rowof openings to reinforce the shell, a row of blades at the exterior ofthe shell, and shanks on theblades extending through said openings andsecured to said ring tomount the blades on the rotor.

14. A compressor rotor comprising a hollow sheet metal rotor shellhaving axially spaced rows of circumferentially 'spaced openings, ringss ecured to the interior of the shell at said rows of openings toreinforce the shell, rows of circurn-i ferentially spaced blades at `theexterior of the shell, shanks on the blades extending through saidopenings, and means securing the shanks to said rings to attach theblades to the rotor.

15. A compressor rotor comprising a thinwalled rotor shell of generallytubular form having at least one row of circumferentially spacedopenings in its peripheral wall, a reinforcing ring secured to theinterior of the shell at said row of openings, a row of hollow blades atthe exterior of the shell, shank fittings bonded to the interiors of theblades and extending into said openings, and means for securing saidfittings to the ring to attach the blades to the rotor. 16. A compressorrotorcomprising a thinwalled rotor shell of generally tubular formhaving at least one row of circumferentially spaced openings in itsperipheral wall, a reinforcing ring secured to the interior of the shellat said row of openings, a row of hollow blades at the exterior of theshell, shanks extending into said openings, parts on the shanksconforming to and bonded with the interiors of the blades, and means forsecuring the shanks to said ring.

17. A compressor rotor comprising a thinwalled rotor shell, areinforcing ring on the shell, a row of hollow blades for the rotor,shanks for the blades including tongues conforming to and bonded withthe interiors of the blades, and means for securing the shanks to saidring.

18. A compressor rotor comprising a thinwalled shell, a reinforcing ringsecured within the shell to reinforce the same, there being a row ofopenings passing through the wall of the shell and said ring, a row ofblades at the exterior of the shell, shanks projecting from the rootends of the blades and received in said openings, and screwv threadedmeans securing the shanks to the ring.

19. A compressor rotor comprising a thinwalled shell, a reinforcing ringsecured within the shell to reinforce the same, there being a row ofopenings passing through the wall of the shell and said ring, outwardlyfacing seats on the ring at the openings. a 20w of bladesat the exteriorof the shell. Shanks on the blades received in said openings, parts onthe shanks bearing in-v wardly against said seats, and screws threadedinto the Shanks and clamping outwardly against the ring to secure theblades to the ring. 5 20. A compressor rotor comprising a thinwalledrotor shell, a thin-walled reinforcing ring ,secured to the interior ofthe shell, there being a row of openings passing substantially radiallythrough the wall of the shell and ring, a row of4 l0 hollow blades atthe exterior of the shell, vShanks received in said openings, parts onthe Shanks conforming to and bonded with the interiors of the blades,and screws threaded into the shanks to cooperate with the ring to securethe blades i3 thereto, the shanks and screws having passages for placingthe interiors of the blades in communication with the interior of theshell.

21. A compressor rotor comprising a thinwalled shell, a reinforcing ringsecured within go the shell to reinforce the same, there being a row ofopenings passing through the Wall of the shell and said ring, outwardlyfacing seats on the ring at the openings, a row of blades 'at the ex-Ving at least one row of circumferentially spaced openings in itsperipheral wall, a reinforcing ring secured to the interior of the shellat said row of openings, a row of hollow sheet metal blades at theexterior of the shell, precision-cast shank fittings extending into saidopenings and having parts conforming to and bonded with the interiors 40of the blades, and means for securing the fittings to the blades. Y

23. In an axial compressor, a hollow sheet metal rotor shell having arow of circumferentially spaced openings, a ring secured to the interiorof the shell 'at said row of openings to reinforce the shell, the ringhaving a peripheral flange of approximately the same thickness as theshell wall engaging thereagainst and secured thereto, a row of blades atthe exterior of the shell, and shanks on the blades extending throughsaid openings and secured to said ring to mount the blades o the rotor.

` NATHAN C. PRICE.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS 0 Number Name Date 752,340 Holzwarth Feb. 16, 19041,014,350 Wales et al Jan. 9, 1912 1,967,962 Metten July 24, 19342,213,940 Jendrassik Sept. 3, 1940 5 2,241,782 Jendrassik May 13, 19412,314,058 Stalker Mar. 16, 1943 2,337,619 Miller Dec. 28, 1943 2,344,835Stalker Mar. 21, 1944 2,405,768 Stalker Aug. 13, 1946 FOREIGN PATENTSNumber Country Date 20,818 Great Britain Sept. 19, 1906

